ECOLOGICAL CHEMISTRY AND ENGINEERING S

Vol. 18, No. 1 2011

Marek STELMACHOWSKI1

UTILIZATION OF GLYCEROL, A BY-PRODUCT OF THE TRANSESTRIFICATION PROCESS OF VEGETABLE OILS: A REVIEW METODY ZAGOSPODAROWANIA GLICERYNY, PRODUKTU UBOCZNEGO Z PROCESU TRANSESTRYFIKACJI OLEJW ROLINNYCH: PRZEGLD LITERATUROWY Abstract: The use of biodiesel is expected to grow in the future due to the environmental policy of the EU. The increase in the production of biodiesel has resulted in a glut of glycerin that has led to a precipitous market price drop. At present glycerol is in surplus but in the near future could become a waste problem. It may create a barrier for the development of this industry branch and reduce biodiesel applications as well. This glut also indicates that known technologies for the utilization of glycerol have many disadvantages and that their efficiency and profitability are too low. The utilization of waste glycerol may be carried out by feedstock recycling into energy carriers (hydrogen, syngas and methane), or it may be converted into other chemicals (eg, acrolein, epichlorohydrin, ethers, esters and alcohols). As hydrogen is a clean energy carrier, the conversion of glycerol into hydrogen is the best glycerol use. Catalytic steam reforming, aqueous reforming, autothermal reforming, pyrolysis, gasification, photo-conversion and bioconversion of glycerol may be considered to solve the glycerol surplus problem. This analysis indicates that the (aqueous/steam) reforming and/or photocatalytic conversion of glycerol have become the best processes for the utilization of glycerol from an economic and environmental point of view. Keywords: glycerol, hydrogen, pyrolysis, gasification, bioconversion, photocatalysis, reforming. Introduction The limited oil reserves on Earth, the increasing price of a barrel of oil (which is sometimes higher than $100) and the need to reduce CO2 emissions resulting from the combustion of fossil fuel have resulted in the search for new technologies that can produce new types of liquid fuels and new additives for traditional liquid fuels. Transesterification of vegetable oils (palm, soybean, rapeseed and many others) is a method of producing biodiesel, which is a mixture of esters (methyl or ethyl) of higher fatty acids (FMEs). Bioethanol (also produced from biomass) and FMEs are the most popular additives in

10 traditional fuels. The European Union recommended in the 2003/30/EC Directive that the fraction of biofuels in conventional fuels should at least be 5.75% wt. in 2010 and 20% in 2020 [1]. Therefore, the production of biofuels has recently dramatically increased. Statistical data from different sources differ to a small extent. In 2004, the production of biodiesel in EU countries was approximately 1.93 million tons(Mg), while it amounted to 4.89 million tons in 2006 (in some reports, this value was given as 3.1 million tons). In 2006, it was also predicted that the production of biodiesel in 2012 in the EU countries could be more than 10 million tons [2, 3]. In fact, in 2008, the amount of biodiesel produced was 7.8 million tons [4]. A production of 20.9 million tons has been reported for 2009, and only two countries, Germany and France, produced more than 7.7 million tons. However, the European Biodiesel Board (EBB) official statistical data for 2009 were based on the declared capacity of existing processing plants [4]. Thus, the actual amount of biodiesel produced was probably lower. In the USA, the production of biodiesel increased from 0.5 to 250 million gallons(3.79 dm3) between 1999 and 2006 [5]. Similar results could probably be observed in China, Canada and other countries with high transport fuel demands.

Fig. 1. Predicted and actual production of rapeseed, FMEs and glycerol in Poland in relation to the National Indicative Objective (NIO) [4, 6, 7] Utilization of glycerol, a by-product of the transestrification process of vegetable oils: a review

11 According to EBB estimates, the production capacity of Poland was approximately 0.58 million tons in 2009 [4]. However, the actual production was probably 10 to 20% lower. It is predicted that the annual amount of manufactured FMEs will be around 1.5 million tons in the coming years. Figure 1 represents the actual output growth between 2005 and 2008 and the growth predicted from 2008 to 2020 for rapeseed, bioesters and glycerol. As can be seen, the current fraction of biofuels in transport fuels in Poland is not close to the value required by the EU transport and fuel policies, ie, 5.75%. A value of 5.75% could not be achieved in Poland because the supply of rapeseed oil esters is below 0.60.65 million tons/year [8-10]. There are also significant differences between the objectives set by the EU policies and the results expected by the National Indicative Objective (NIO) (especially before 2008) for the fraction of biofuels in the structure of energy consumption in Poland. The EU environmental policy, which assumes that the combustion of fossil fuels is responsible for climate changes, will be applied in the future even if this assumption has not yet been confirmed; thus the profitability of the policy is undetermined and many researchers have questioned the methodology and quality of the tools used for assessing the effects of fossil fuel combustion. However, from the other side, the increase of the price of an oil barrel and a higher biodiesel demand may increase the profitability of the production of biodiesel. The evaluation of the economic and technological aspects of biodiesel production in Poland and the EU is not simple. It is essential to assess whether: - it is possible to use the required acreage of agricultural land that is suitable for the cultivation of rapeseed (which in Europe would be the basic raw material for the manufacture of bioesters), - the acreage of other crops cultivation can be exempt for the cultivation of rapeseed; it is not certain that ethical aspects can be excluded from this assessment; however, as these aspects cannot be quantified, they will be ignored in the rest of the present study. The following factors have also to be determined: - the optimal size of the acreage of the rapeseed cultivation, - the optimal volume of the oil-producing facilities and their distance from the cultivation area to take into account the energy costs related to transport, - the technology of the transesterification process, - the optimal scale of the industrial plant of biodiesel production, - the selection of methods and processes that use the wastes and by-products, which increase the profitability of the whole process. It should be also taken into account that rapeseed is cultivated seasonally and harvested during the 2-3 months of summer in Europe. The seeds must therefore be stored in a manner that allows the facilities to work for the next 10 to 11 months. Some preliminary analyses regarding Polands problems were reported by Jerzykiewicz et al [11] and Korytowski and Inwolski [12]. Both groups mentioned issues related to the development of waste and by-products of different technologies. Further economic analyses are indispensable to promote the use of biofuels. These analyses should demonstrate that the production of biofuels is profitable for both the farmers and the processing plants. Several wastes and by-products are generated during the production of FMEs from rapeseed. A simplified diagram of the whole process is presented in Figure 2. Until now, the utilization of straw and oilseed cake has not generated significant problems when rapeseed has been cultivated for a traditional use, ie, the manufacturing of Marek Stelmachowski

12 edible oil. Straw and oilseed cake have been used for the manufacturing of protein feed and solid fuels (pellets, briquettes) to produce green energy. However, the use of the whole area of agricultural land dedicated to the cultivation of rapeseed to achieve the required fraction of bioesters will result in a large amount of these two by-products, which will be problematic. New techniques have to be developed to efficiently use straw and oilseed cake. For instance, the components could be used to produce amino acids, proteins and fats, among others.

Fig. 2. Simplified diagram of the production of FMEs based on the NBB data Glycerol, a by-product of the production of bioesters The most important by-product obtained during oil transesterification is crude glycerol. This phase contains glycerol, unprocessed methanol and smaller amounts of other substrates and by-products. The mass of crude glycerol is equal to approximately 12% wt. of the obtained mass. The content of pure glycerol in the glycerin phase is about 6070% wt.

Fig. 3. Traditional glycerol uses with average worldwide values. In Poland, the data are a little different Utilization of glycerol, a by-product of the transestrification process of vegetable oils: a review

13 The estimated worldwide glycerin demand in 2006 was between 93 000 and 95 000 Mg. The demand was slightly higher in subsequent years because of economic growth. The traditional uses of glycerin are shown in Figure 3. The increase of the production of biodiesel and the resulting excessive amount of glycerol produced by the transesterification of oils have led to considerable lower market prices of glycerol. Figure 4 shows the fluctuations of the glycerol price and the correlation of the price with the volume of manufactured glycerol (derived from the production of biodiesel) in Europe between 1996 and 2008. Glycerol prices reached 300400 $/Mg but there were periods when it dropped to 50100 $/Mg.

Fig. 4. Fluctuations of the glycerol price in Europe [3, 4, 10] A further increase in the production of biodiesel and glycerol will result in a further decrease of the glycerol price in the long term. This will also decrease the profitability of the production of rapeseed and bioesters unless new glycerol utilizations are found. The glut of glycerol may slow the development of this industrial branch and reduce the application of biodiesel. This glut also indicates that the technologies using glycerol present many disadvantages and that their efficiency and profitability are too low. The simplest utilization method of crude glycerol is its combustion, which is an advantageous method as it does not require any purification, which would increase the cost of the method. However, this process is not easy from a technological point of view. The following stoichiometric equation corresponds to the combustion of glycerol: kJ/mol 1655 O 4H 3CO 3,5O (OH) H C2 2 2 3 5 3 + + (1) The heat of combustion is two times lower than that of fossil fuels but is comparable to the heat of combustion of most types of biomass (eg, wood, straw, oilseed cake, bark, chips, willow and sawdust). The calorific value is lower due to the relatively large amount of water that is obtained. Moreover, water renders the combustion of glycerol very difficult because it leads to the blanketing of the flame at the burners and the formation of large quantities of carbon black. In practice, it is essential to perform co-combustion of glycerol with other Marek Stelmachowski

14 liquid fuels using special burners. The co-combustion of the glycerol/glycerol phase and oilseed cake and/or straw has not yet been carried out in a large scale.

Fig. 5. Methods using waste glycerol obtained by transesterification of vegetable oils Various methods have been reported in the literature for the utilization of glycerol, some of which have already been implemented on an industrial scale. However, it seems that at least some well-known technologies lead to the manufacturing of traditional products that are usually obtained by other technologies. The technologies that use glycerol may result in a lower profitability for the production of such traditional products. The methods using waste glycerol can be categorized based either on the obtained products (energy carriers) or the technology. The different technological methods are further discussed in the chapter. Figure 5 gives an overview of different methods using glycerol. Utilization of glycerol, a by-product of the transestrification process of vegetable oils: a review

Fig. 6. Reaction pathways and products obtained by pyrolysis of glycerol at atmospheric pressure Chaudhary and Bhakshi [13], Valliypana et al [14] and Fernandez et al [15], Stein et al, [26] among others, have studied the pyrolysis and gasification of glycerol at 650800C under ambient pressure. Dou and Dupont investigated the kinetics of the pyrolysis of crude Marek Stelmachowski

16 glycerol (from the biorefinery of D1-Oils Ltd., Middlesborough, UK) using thermogravimetry [17]. They designated the kinetic constants for three simple kinetic models and distinguished three stages for the process. Reaction pathway diagram of the process proposed by the authors is presented in Figure 6 and experimental results obtained by Valliypan and Fernadnez are summarized in Table 1. A few reports in the patent literature state that the pyrolysis of glycerol for generation of hydrogen is not the technically and economically most advantageous solution. Reforming of glycerol The reforming of glycerol to hydrogen and/or carbon monoxide and hydrocarbons has been widely studied [18-35]. The following stoichiometric equation corresponds to the reforming of glycerol [18]: ........ eCH dH O cH bCO aCO yO O xH O H C4 2 2 2 2 2 8 3 + + + + + + + (2) Depending on the heating source and the reagents, the process can be operated as a steam reforming in the gas phase (with or without catalysts) or in the aqueous phase (aqueous-phase reforming; APR) and as an autothermal process in the aqueous phase. The most effective catalysts for the reforming of oxygenated hydrocarbons are based on Group VIII metals because these metals generally facilitate the breaking of CC bonds. Silica or (Al2O3)-supported nickel, cobalt, nickel/copper, platinum (and other metals: Rh, Ir, Ru, Zr, Ce, La) are the most commonly used catalysts [18-22]. The steam reforming of glycerol (x > 0, y = 0) is described by the following equation:

2(gas) 2(gas) (gas) 2 3(gas) 8 33CO 7H O 3H O H C +

+ kJ/kmol 345 H298 = (3) In view of the stoichiometry of the reaction, the maximum amount of hydrogen that can be produced is 7 moles of hydrogen per mole of glycerol. However, as it is an equilibrium reaction, the maximum yield is smaller. Douette et al studied the reforming of pure glycerol and a glycerol phase (from Pacific Biodiesel, Honolulu, HI, USA) in a laboratory-scale tubular reactor using a nickel catalyst [18]. A yield of 4.6 moles of H2 per mole of glycerol was obtained at 760C. The drawbacks of this method are high energy inputs and a quick deactivation of the catalysts. Adhikari et al also performed a comparative thermodynamic analysis and experimental studies [5, 19-21]. The thermodynamic analysis of the process showed that a high temperature (T > 420C), low pressure (0.1 MPa) and a high ratio of water to glycerol (9:1) led to a high conversion of glycerol into hydrogen (the conversion to methane was minimized and the conversion to coal did not occur). The authors confirm also the essential role of the water gas shift reaction. Other, essential thermodynamic studies of this process were performed also by Kunkes et al [23] and Wang et al [24]. The reforming of glycerol can also take place in the aqueous phase (APR). For x = 0, y = 0 and a C:O ratio of 1:1 for organic compounds, the reforming of glycerol is given by the following stoichiometric equation:

2 3 8 34H 3CO O H C +

(4) Under the above-mentioned conditions, the reaction is accompanied by the water gas shift reaction Utilization of glycerol, a by-product of the transestrification process of vegetable oils: a review

17

2 2 2H CO O H CO +

+ (5) The advantages and drawbacks of the method were discussed by Davda et al [25], who performed thermodynamic and kinetic analyses of the conversion of oxygen derivatives into hydrogen and/or alkanes. Several metals supported on various materials were investigated to optimize the yield and selectivity of the hydrogen production. It was claimed that catalysts based on Pt and Ni-Sn alloys are the most promising materials for hydrogen production and that the catalytic activity of metals is Pt ~ Ni > Ru > Rh ~ Pd > Ir. The selectivity depends on various factors such as the nature of the catalytically active metal, the support, the pH of the solution, the feed and the process conditions. Davda et al also proved that the method can be highly efficient. In their opinion, the reforming reaction under APR conditions: - reduces the amount of energy required, the substrates and products do not need to be vaporized, - makes the process more safe because the products are non-toxic and non-flammable, - can lead to decreased CO production and increased hydrogen production because the water gas shift reaction is favorable in the process conditions, - hydrogen and CO can be easily and effectively purified using membrane technologies because the pressure of the effluent gas is high (1.55 MPa), - is performed at temperatures that minimize undesirable reactions, - gives the possibility of performing the process in a single step at low temperature. Wen et al also investigated the reforming of glycerol in the aqueous phase [26]. They investigated the activity and stability of catalysts that are based on Pt, Ni, Co or Cu zeolites. They also investigated the effects of the process conditions, catalyst type and catalyst support on the production of hydrogen. Some of the obtained results are presented in Table 2. Table 2 Conversion of glycerol into hydrogen under APR conditions [26] Product composition Carbon conversion H2 CO2 CO Alkanes Productivity Catalyst [%] [% mol] mmol/[min gcat.] Pt/Al2O3 18.9 69.8 24.5 0.05 5.8 572.2 Pt/SiO2 10.8 71.8 25.8 0.06 2.3 369.4 Pt/act. carbon 17.2 69.6 27.1 0.08 3.1 307.7 Pt/MgO 13.8 79.9 18.2 0.10 1.9 431.9 Pt/HUSY 22.0 71.8 21.4 0.07 6.8 337.0 Pt/SAPO-11 13.3 72.8 24.4 0.09 2.6 222.1

In several articles, [5, 21, 24, 27-32], comparative thermodynamic analyses were performed. In most cases, it was concluded that it is necessary to purify the crude glycerol (glycerol phase) before using it in the process. It seems that a further and more detailed discussion of all the methods of conversion of glycerol to hydrogen will be presented. In April 2009, the Linde Group announced the construction of a demonstration plant for the conversion of glycerol for mid-2010. However, the method and technology that would be used for the conversion of glycerol were not revealed. In other words, it is unclear whether a low-temperature pyrolysis (400600C) to liquid products, a high-temperature Marek Stelmachowski

18 pyrolysis to gaseous products or a steam reforming (under APR conditions) to hydrogen will be performed. Processes (eg, reforming and oxidation) performed in supercritical water have been perceived as very interesting. Adhikari et al [20], Byrdet al [33], Xu et al [34] and Chainkala et al [35] have experimentally studied such processes. The product obtained in these processes consists of hydrogen, carbon dioxide and methane. It is noteworthy that the process conditions are rather drastic in this case (T > 420C, P > 24 MPa). This will certainly limit the applicability of the technology at a large industrial scale for economic reasons. Photocatalysis as a method to convert glycerol into hydrogen The idea of using a photocatalytic process for the conversion of glycerol is based on results obtained for the degradation of organic pollutants in wastewater and the conversion of alcohols (eg, ethanol) into hydrogen. Patsoury et al, Kondarides et al, Veykios et al and Strataki et al [36-38] have reported results obtained during the degradation of organic pollutants, ie, simple organic alcohols, organic acids and aldehydes. The results indicated that glycerol, can be converted into hydrogen in the presence of photocatalysts under mild conditions. The reaction is given by eq. (9):

2(gas) 2(gas) (liquid) 2 3(liquid) 8 33CO 7H O 3H O H C + + (9) It is endothermic and proceeds under ambient conditions. The energy is obtained from electromagnetic radiation. Specifically, in the laboratory, the energy source is a lamp, eg, a mercury lamp (for UV) or a xenon lamp (simulating sunlight). The catalysts for the reaction are based on the most widely used photocatalyst, TiO2, which is mainly modified with gold, palladium, platinum or copper. Different laboratory reactors were used in the investigations. Tank batch photoreactors with a stirrer and in which the catalyst was suspended in a liquid (glycerol solution in water) were mainly used.

Kondarides et al, Kondaridiesand & Daskalaki, Li and others [39-41] studied the decomposition of water with simultaneous oxidation of various organic substances (ie, ethanol, glycerol, and sugars) at ambient temperature and under atmospheric pressure in the presence of a Pt/TiO2 catalyst. Their main goal was to determine the influence of various process parameters on the efficiency of the process. Bowker et al conducted experiments on Utilization of glycerol, a by-product of the transestrification process of vegetable oils: a review

19 the decomposition of glycerol to produce hydrogen with a TiO2 catalyst modified with palladium and gold [42]. Bowker et al also presented their own proposal of a six-stage model for the mechanism of the photocatalytic conversion process. Gombac et al performed a comparative study of the photocatalytic conversion of ethanol and glycerol into hydrogen using a CuOx/TiO2 catalyst [43]. The influence of the catalyst preparation and the nature of the copper ions that are present on the surface of the TiO2 catalyst were also investigated. Wu et al investigated the role of copper (in various forms) in the photocatalytic conversion of glycerol into hydrogen in the presence of a CuOx/TiO2 catalyst [44]. They found that high concentrations of Cu(I) led to higher yields of hydrogen than Cu(0). The inhibitory role of Cu(II), initially reported by Li et al, was confirmed. Table 3 summarizes all the results obtained by the different groups. A new generation of titanium semiconductors with a photocatalytic activity that includes the visible spectrum (> 400 nm) is currently being developed. With these catalysts, the energy inputs are reduced. Photocatalysts that are active over the entire visible spectrum are obtained by different preparation methods [45]: (A) modification of by addition to a basic support noble or other metalsa (Anpo, [46]), (B) preparation of reduced forms of TiO2-x (Nakamura et al [47]), (C) sensibilization of TiO2 using dyes (Chatterjee, Mahata [48]), (D) sensibilization of TiO2 with semiconductors (Hirai et al [49]), (E) doping of a base catalyst with a non-metal (N, S, C, B, P, F, I) [50-53], (F) doping of a catalyst with luminescent substances (Feng [54]). The recovery process of spent catalysts from the solutions may limit the use of heterogeneous photocatalysis in industrial processes. Immobilization of the catalyst in the reactor and/or separation of the catalyst particles from the purified liquid are required before disposal of the solution [55]. By solving this problems the method could be used for the treatment of glycerol waste in an efficient manner. Biochemical conversion of glycerol into energetic raw materials Sabourin-Provost and Hallenbeck studied the photofermentation of a crude glycerol fraction into hydrogen in the presence of Rhodopseudomonas palustris bacteria (CGA009, wild-type; CGA750, NIF-) [56]. Six moles of hydrogen were obtained per mole of glycerol. The presented results are only preliminary. Seifert and Waligorska et al studied a periodic glycerol fermentation in which the main product was hydrogen [57]. A glycerol conversion of about 75% was achieved, and 0.41 moles of hydrogen per mole of glycerol were obtained; the maximum rate of hydrogen production was 0.15 g H2/(dm3 h). It was also found that the addition of glycerol increased the production of biogas (methane) during the biodegradation/fermentation of organic wastes. Siles Lopez et al conducted one of the best studies on the anaerobic bioconversion of glycerol into biogas (methane was a main component) [58]. Using an aqueous solution of glycerol and sewage sludge as the raw material, they achieved a conversion of almost 100% of the stock (ie, the organic loading). The methane yield amounted to nearly 0.14 kmol of CH4/kg of reacted glycerol under ambient conditions. This result is better than that obtained for the conversion of glycerol into hydrogen; additionally, the reaction rate was higher. The bioconversion of glycerol (and organic wastes) into hydrogen has only been studied in the laboratory. The applicability of the method to industrial processes has not yet been evaluated. Marek Stelmachowski

20 Product recycling of glycerol Selective catalytic oxidation As can be seen in Figure 5, not a large number of products can be obtained by selective oxidation of glycerol. Demirel et al have investigated the oxidation of glycerol in the liquid phase and in the presence of a catalyst and compared their results with those reported in the literature [59, 60]. The effects of the catalyst particle size, temperature, pressure and concentration of NaOH in the solution on the selectivity (to glycolic acid, tartronic acid and other products) and the reaction rate were investigated. The maximum glycerol conversion that was obtained was ~50%. Studies performed with other catalysts have also been reported. Thus, Srivastava et al have investigated the kinetics of the oxidation of glycerol in the presence of a ruthenium catalyst [61]. Bianchi et al evaluated the selective catalytic oxidation in the presence of mono- and bimetallic catalysts [62]. Ketchi et al and Demirel studied the activity of carbon-supported catalysts and of a bimetal AuPd catalyst in the oxidation of glycerol and carbon monoxide [63]. Liang et al [64] and Prati et al [65] investigated the same reaction in the presence of a platinum catalyst and a gold catalyst (in the form of nanoparticles), which was supported on either activated carbon or TiO2, respectively. On the other hand, Zope and Davis studied the influence of process parameters on the oxidation of glycerol with molecular oxygen in the liquid phase and in the presence of a Au/TiO2 catalyst [66]. The reaction in the presence of mono- or bimetallic catalysts based on noble metals was supported) on activated carbon or TiO2 occurs at a relatively low temperature and with a conversion of up to 50%.

Fig. 5. Possible reaction pathways and products obtained by selective catalytic oxidation of glycerol Chemical conversion of glycerol into other chemical products Most scientific studies focused on the possible manufacturing of useful products and semi-products that are different from those obtained by oxidation or reforming of glycerol. Modifications of known processes and new methods have thus been proposed. Zheng et al reviewed the different methods [67]. The following products can be obtained: - acrolein (CH2=CH-CHO) obtained by dehydrogenation, Utilization of glycerol, a by-product of the transestrification process of vegetable oils: a review

Fig. 6. Methods of conversion of glycerol into useful products (excluding selective oxidation) Figure 6 summarizes the different possible reaction pathways and products. Most of the studies focused on the dehydration of glycerol into acrolein (eg, Corma et al [68]), the synthesis of alcohols, particularly 1,2-propanediol (eg, Guo et al [69]), and the production of additives for fuels (eg, Frusteri et al [70]). The yield, selectivity and glycerol conversion are very different for different products, as discussed below. Watanabe et al reported the results of the synthesis of acrolein from [71]; a glycerol conversion of 90% and selectivity for acrolein of 80% were achieved. Tsukuda et al also studied the synthesis of acrolein from glycerol [72]. The aim of the study was to optimize the composition of the catalyst. The best results were obtained with the highest temperature and a Q10-SIW-30 (H4SiW12O4024H2O) catalyst. A glycerol conversion of almost 100% and selectivity for glycerol of over 85% were achieved. Kijenski et al from the Industrial Chemistry Research Institute (ICRI) in Warsaw (Poland) studied the dehydration of glycerol in a fixed-bed flow reactor in the presence of various catalysts: Al2O3, SiO2, Al2O3, TiO2 and the same compounds impregnated with H2SO4, H3PO4 and H3Mo12O40P [73]. The conversion to acrolein and allyl alcohol was the highest: it was 58 mol% between 300 and 320C. Corma et al also studied the conversion of glycerol to acrolein in two different reactors: a fluidized bed reactor similar to an FCC and a fixed-bed reactor [68]. It was found that lower temperatures favor the conversion into acrolein (yield of 55 to 62%) and higher temperatures favor the conversion into acetaldehyde. Atia et al investigated and discussed the conversion of glycerol into acrolein in the presence of various catalysts of formula [Xn + Y12O40]8-nxH2O Marek Stelmachowski

22 (where X P, Si, B, Y Mo, W, V and "n" is the oxidation number of X) [74]. With all catalysts, acrolein was the main product; the selectivity for acrolein was 75% at 275C for X Si and Y W. Suprun et al studied the influence of acid catalysts on the selectivity of the dehydration reaction of glycerol to acrolein [75]. The highest yield of acrolein (ie, 72%) was obtained for the zeolite SAPO-34. Ulgen and Hoeldrich studied the dehydration of glycerol into acrolein on a WO3/ZrO2 catalyst [76]. The highest selectivity was ~75% with a total conversion of glycerol. Kim et al investigated the same reaction using zeolites as catalysts [77]. The results were similar to those previously discussed. Wei and Suppes obtained acrolein in 67% yield with a glycerol conversion of 99% and a selectivity of 84% [78]. Frusteri et al prepared fuel additives by etherification of glycerol with tert-butyl alcohol [70]. Both the glycerol conversion and the selectivity were high and exceeded 93%. Kijenski et al also studied the conversion of glycerol into its tert-butyl ether derivative [79]. A glycerol conversion of 50% was achieved and a high selectivity to ethers was obtained for long reaction times. Guo et al investigated the reduction of glycerol into 1,2-propanediol with a bifunctional Co/MgO catalyst [69]. The maximum glycerol conversion was about 55% and the selectivity for the desired product did not exceed 42%. Akiyama et al also studied the conversion of glycerol into 1,2-propanediol (1,2-PDO) [80]. A glycerol conversion of 100% was obtained with copper catalysts (Cu/Al2O3). A selectivity of 75% was obtained at 190200C and under a partial hydrogen pressure of 0.1 MPa. Huang et al investigated the conversion of glycerol into 1,3-propanediol (1,3-PDO). The main goal of this study was to demonstrate that the conversion of glycerol into the desired products occurred in one step. The obtained results confirmed the hypothesis [81]. Similar studies were carried out by Alhanashi et al [82] and Ma and He [83]. Luo et al studied the direct chlorination of glycerol to form 1,3-dichloropropanol (DCP), which is a raw material in the production of epichlorohydrin [84]. The goal of the study was to model the reaction kinetics. A very high glycerol conversion (at T = 90110C) was obtained. The direct chlorination of glycerol (by gaseous hydrogen chloride) to form DCP was also investigated by Song et al in a batch reactor [85]. The conversion of glycerol was close to 100% and the selectivity to DCP was ~90%. Santacesaria et al also investigated the chlorination of glycerol with gaseous hydrogen chloride under various conditions [86]. 1.3-dichlorohydrine was obtained in more than 95 mol %. Sakthivel et al reported the esterification of glycerol with lauric acid (dodecane acid) in supercritical carbon dioxide and in the presence of catalysts based on a selected group of molecular sieves [87]. A high glycerol conversion and a high global selectivity to three esters of lauric acid were achieved. However, the conditions used (10 MPa, 150C, reaction time = 18 h) would be problematic on a larger scale. Maris et al studied theoretically and experimentally the hydrogenolysis of glycerol to ethyl and propyl glycols in the presence of bimetallic, bifunctional PtRu/Cact and AuRu/Cact catalysts [88]. The study focused primarily on the role of the catalysts. More detailed results were obtained by Feng et al in the presence of a ruthenium catalyst [89]. Feng used a partial hydrogen pressure of 5 MPa, a temperature of 180C, a glycerol concentration in water of 20% wt. and a reaction time of 12 h. The glycerol conversion was 90.1%, the selectivity for 1,2-propanediol was 20.6% and that for ethylene glycol 41.3%. The same reaction was also Utilization of glycerol, a by-product of the transestrification process of vegetable oils: a review

23 studied by Marinoiu et al [90] and Chiu et al [91]. They focused on the optimization of process parameters such as the glycerol concentration, pressure, temperature and residence time to obtain a selectivity for propylene glycol that was greater than 90%. The evaluation and assessment of methods for the conversion of waste glycerol to other chemicals is relatively difficult. On one hand, product recycling of glycerol should be favored as it leads to useful substances, allow to keep elements and effective (or primary) energy in the global world cycles and prevent the uncontrolled dispersion of waste energy. Therefore, this type of waste management method is the best way to control and prevent environmental pollution and is the best instrument of environmental policy. On the other hand, the application of the aforementioned methods in the recycling of glycerol is limited by, many essential factors: - the process conditions (eg, high pressure and high temperature), - the low selectivity and therefore a large range of products, - the low glycerol conversion (in many cases), - necessity of maintaining of the process conditions in a small range of variability and purification of the input material. The application of the methods will also be limited by the market profile of the obtained products, which are often obtained with other technologies. Thus, a thorough market and economic analysis must be conducted. Biochemical conversion of glycerol into other products The conversion of glycerol into other useful chemicals using microorganisms has also been investigated. Examples of such investigation include: - The use of glycerol as a carbon source for the production of -carotene. Mantzouridou, Naziri and Tsimidou replaced glucose with glycerol in the production of -carotene in the presence of Blakeslea trispora [92]. The use of a crude glycerol phase instead of pure glycerol did not inhibit the reaction. - Microbiological conversion of glycerin to 1,3-propanediol. Deckwer has shown that glycerol can be converted into 1,3-propanediol with Klebsiella and Citrobacter Clostridia by fermentation [93]. The costs and feasibility of the process were also assessed from an industrial point of view. - The use of glycerol as a source of organic carbon in the fermentation processes. Celik et al investigated by fermentation [94] the synthesis of various other organic compounds (organic acids of high molecular weights) using microorganisms. - Athalye et al [95] and Yu et al [96] proposed the conversion of glycerol into ethanol using Saccharomyces cerevisiae. The results indicate that the methods are industrially feasible. However, it must be considered that biochemical reactions are usually slow. This means that even though the process conditions are good, the kinetics of the process will result in large bioreactors. It is still very difficult to realistically assess the bioconversion of crude glycerol into useful products because not enough studies have been completed. Conclusions The production of ecological fuel components (bioesters, bioethanol) that are manufactured from biomass and that are additives for traditional motor fuels (which are Marek Stelmachowski

24 obtained from fossil fuels) has recently drastically increased due to the EU environmental policy. For instance, the production of FMEs has been increased by at least ten-fold over the past five years. The EU environmental policy is based on: - the growing need for independence, even partially, from external energy sources and - the assumption that carbon dioxide (from anthropogenic sources, especially those produced during the combustion of fossil fuels) is responsible for the increase in air temperature and climate changes. The above-mentioned assumption has not been scientifically proven (eg, on the basic of thermodynamic considerations and/or energy balances calculations) and is not economically justified. Nevertheless, it seems that there is no return from once established way. Excellent methods for the production of FMEs and utilization of all of the by-products and wastes (eg, straw, oilseed cake and crude glycerol) that are generated during the production process are therefore required. The methods should be profitable at all stages of the process: during the cultivation of rapeseed (agriculture), in the oil industry (pressing of the oil and its transesterification) and during the production of fuels (for the petroleum industry). This will only be the case if all wastes and by-products are converted into useful materials. At present, straw and oilseed cake are either combusted or used as fodder or fodder ingredients with sufficient profitability. However, the utilization of crude glycerol that is generated during transesterification remains problematic. Based on the articles reviewed and discussed herein, it may be stated that the well-known and available technologies of the utilization of glycerol and the obtained products decreased profitability of such processes because of the huge amounts of glycerol that are produced. These large amounts have resulted in a dramatic drop of the glycerol price on the market, which means that the traditional products obtained from glycerol has met with the barrier of limited demand. There are only a limited number of patent and scientific publications on the new conversion methods of glycerol into useful products or energy carriers. The investigations of innovative technologies are in most cases in the preliminary phase of research. Thus, the situation is economically difficult in European countries because they need new advanced methods of using glycerol that are not only industrially feasible but also profitable. These methods should allow the production of chemical products from a low-cost material, ie, glycerol. The apparatus used must also be inexpensive, and a market and economic analysis must first be performed. Currently, the most feasible and promising methods are the conversion processes of glycerol into acrolein, epichlorhydrin, ethers, alcohols and esters. However, the demand for the obtained products might reach a limit too. It is therefore also necessary to develop new methods for the conversion of raw bio-wastes into energy carriers, ie, third-generation biofuels by various technologies based on feedstock recycling. Such methods have not really been developed yet; current methods do not have optimal economic and technological parameters, which cause a progressive price drop of glycerol that has been observed on world markets. However, it seems, that the conversion of glycerol into hydrogen (or synthesis gas) is the most promising method, as the market for hydrogen (syn-gas) should be unlimited in the future. The question is, which of the investigated and developed processes and technologies may be the most profitable one? Good process parameters have not yet been found for the gasification and pyrolysis of glycerol (to synthesis gas and/or hydrogen and hydrocarbons). The most-investigated method is the reforming of glycerol into hydrogen. Several types of reactions have been Utilization of glycerol, a by-product of the transestrification process of vegetable oils: a review

25 proposed: steam reforming, reforming in the aqueous phase (APR), autothermal reforming and decomposition in supercritical water. It seems that the most promising reactions are those that utilize steam reforming and/or APR. Supercritical conditions may not be suitable for economic reasons. The bioconversion and photoconversion of glycerin into hydrogen are still at a preliminary stage. To properly assess a method, the following two points should be considered: First, the method should have little impact on the environment. The least energy-consuming methods, ie the bioconversion and photoconversion of glycerin, are the most environmentally friendly methods. Second, the process parameters, ie, the temperature, pressure, reaction rate and hydrogen yield and selectivity must be considered. These parameters affect the reaction time and plant size and thus the profitability of the method. Methods involving high pressures and temperatures (based on chemical reactions well known in other industries, such as steam reforming) are often characterized by higher reaction rates. Low-pressure methods occurring at ambient temperature in the presence of microorganisms (bioconversion) or photocatalysts (photoconversion) are characterized by significantly lower reaction rates (larger reactor volumes and longer residence time). Based on the reaction stoichiometry and equilibrium conditions, glycerol yield and conversion will be similar for both groups. Energy inputs will obviously be lower for low-pressure methods due to the use of radiation energy, ie, solar or artificial of xenon lamps. Thus, these methods may eventually become profitable if new, more efficient photocatalysts are found (in terms of energy used and reaction rate). Expensive methods that are performed at high pressure and temperature (eg, oxidation or reforming in supercritical conditions) are less profitable because operating costs should be low and the raw product and final products are cheap. As mentioned earlier, the photoconversion and bioconversion of glycerol have not been extensively studied. It is therefore difficult to assess the competitiveness of this technique with traditional methods (eg, reforming), which are characterized by high reaction rate and great industrial experience in exploiting of them.The optimization of the composition of the catalysts for photoconversion, the proper selection of microorganisms for bioconversion and the determination of the optimal process conditions for photocatalysis and bioconversion are necessary and currently investigated in many universities and R&D institutes. The profitability of the production of biofuels depends on the management of the wastes and by-products produced during full life cycle of them. The development of new methods for the conversion of glycerol into energy, energy carriers and other useful products is funded by all European countries, indicating its importance. In 2008, three projects on the utilization of waste glycerol were started in the 7th Framework Programme [97]. 1. Sustainable and integrated production of liquid biofuels, bioenergy and green chemicals from glycerol in biorefineries (GLYFINERY), 7th FWP (Seventh Framework Programme), Research area: ENERGY-2007-3.3-02 New uses for glycerol in biorefineries, Coordinator TECHNICAL UNIVERSITY OF DENMARK 2. Reforming of crude glycerol in supercritical water to produce methanol for re-use in biodiesel plants (SUPER METHANOL), Coordinator ENSCHEDE NETHERLANDS, The overall project objective is to produce methanol from crude glycerol, and re-use the methanol in the biodiesel plant. Marek Stelmachowski

26 3. Integrated bioconversion of glycerol into value-added products and biogas at pilot plant scale (PROPANERGY), Coordinator TECHNISCHE UNIVERSITET HAMBURG-GERMANY, This proposal aims at developing an integrated bioprocess to convert technical glycerol from biodiesel production into biogas and two value-added products 1,3-propanediol (PDO) and fertilizer in a biorefinery approach. Unfortunately, fundamental gap in the assessment of the fundamental decisions about increasing of the fraction of biofuels in the global mass of energy carriers as well in the research and development of the described processes can be observed. No real thermodynamic analysis (made for instance on the basis of energy (exergy) calculations) and no energy balances for individual national economies and individual industrial process (in which biofuels are manufactured) have been made to confirm the assumptions on which the implemented environmental policy is based. An exergy analysis of the methods and a realistic assessment of the life cycles of fuels (traditionally produced from biomass) would determine the energy efficiency and environmental impact of the methods and products. It is obvious that the ecological profit of the implementation of biofuels (bioesters, bioethanol) can only be properly assessed if the energy and ecological balances take into account the environmental and production aspects of the necessary materials (eg, fertilizer for rapeseed production), the biofuel production (including the transport of seeds, oils and esters) and the management and utilization of wastes and/or by-products (straw, oilseed cake, glycerol, etc.). An environmental and ecological evaluation has been made for the products, but all important aspects have not been considered. References [1] Directive 2003/30/EC of the European Parliament and of the Council of 8 May 2003 on the promotion of the use of biofuels or other renewable fuels for transport, Official Journal of the European, Union. [2] De Guzman D.: European Union promotes increased biodiesel. Chemical Market Reporter, 267(6)19/2005. [3] Schopf N., Schmidt T. and Sobolski D.: Utylizacja termiczna ciekych paliw biogenicznych - wykorzystanie gliceryny jako taniego paliwa alternatywnego (Utilization of liquid biofuels - glycerol as cheap alternative fuel). SAACKE, XIII Sympozjum Naukowo-Techniczne Chemia 2007, 24-26 styczenia 2007, Pock. [4] Statistical Data from European Biodiesel Board website; http://www.ebb-eu.org/stats.php# [5] Adhikari S., Fernando S. and Haryanto A.: A comparative thermodynamic and experimental analysis on hydrogen production by steam reforming of glycerin. Energy & Fuels, 2007, 21, 2306-2310. [6] Wieloletni Program Promocji Biopaliw lub innych paliw odnawialnych na lata 2008-2014, Dokument przyjty przez Rad Ministrw w dniu 24 lipca 2007 r. Multi-annual Programme for the Promotion of Biofuels or other renewable fuels, approved by Polish Goverment on 2007-07-24. [7] Rocznik Statystyczny Rzeczpospolitej Polskiej. GUS, Warszawa 2009. [8] Gabryszewska M. and Owczuk M.: Synteza kwasu akrylowego a dostpno surowcw odnawialnych do jego produkcji, strona internetowa IChP Warszawa; http://www.ichp.pl/pl/innowacyjna.htm [9] Bocheski C.I., Powaka M. and Bocheska A.: Moliwoci biodiesla w Polsce (Biodiesel in Poland). MOTROL, 2006, 8A, 42-48. [10] Grzybek A.: Uwarunkowania surowcowe produkcji biopaliw, 2008, http://www.oze. szczecin.pl /files/download/58_Uwarunkowania%20surowcowe%20produkcji%20biopaliw.pdf [11] Jerzykiewicz, W., Naraniecki B., Terelak K., Trybua S., Kosno J. and Lukoszek M.: Zagospodarowanie frakcji glicerynowych z instalacji biodiesla (Utilization of glycerol fractions from biodiesel plant). Przemys Chem., 2007, 86(5), 397-402. [12] Kopytowski, J.A. and Inwolski A.: Kompleksowy system wytwarzania ciekych biopaliw z rzepaku jako odnawialnego rda energii, (A complex system for producing biofuels from rapeseed oil as a renewable energy source). Przemys Chem., 2007, 86(3), 195-199. [13] Chaudhari S.T. and Bhakshi N.N.: Steam gasification of chars and bio-oil. Report to Bioenergy Development Program Renewable Energy Branch. Energy, Mines and Resources, Canada, Ottawa, Canada, February 2002, 396-436. Utilization of glycerol, a by-product of the transestrification process of vegetable oils: a review